cmdutil: fix accidental name clash with revrange function
The revrange function was used further up, and when a local variable
is defined with the same name, the earlier call to revrange becomes a
'local variable used before assignment' error.
The clash was introduced in
890ad9d6a169.
commands: fix update's help to be more accurate
Branches can have multiple heads, so it doesn't make sense to speak of "the
head of the current branch". What update really does is try to update to the
*tip* of the current branch.